The following disclosure relates to electrical circuits.
A communication system (e.g., a local area network) allows communication between two or more network devices. FIG. 1 illustrates an example communication system 100 that includes a network device 102 and a network device 104. Network devices 102, 104 include computers, switches, routers, hubs, gateways, and similar devices (e.g., devices having a network interface card in a network). Though two network devices are illustrated in FIG. 1 by way of example, communication system 100 can contain a different number of network devices.
Referring to FIG. 2, communication between network device 102 and network device 104 can be conventionally achieved using a communication line 106, formed by unshielded twisted pairs (UTP) of wires (or cables), and transceivers 108-122, one transceiver positioned at each end of a UTP. For example, four UTPs 124-130 are provided in communication line 106 between network device 102 and network device 104. Hybrid circuits 132-146 (e.g., transformers) can be used at the ends of each UTP 124-130 to control access to a corresponding communication channel for full-duplex bidirectional operation. The combination of a hybrid circuit and a transceiver forms one communication channel. Accordingly, FIG. 2 illustrates four channels of communication, each operating in a similar manner. Each UTP 124-130 is connected to a corresponding transceiver through connectors 148-162.
A common problem associated with a communication system using multiple UTPs and multiple transceivers is noise in the form of interference signals. The interference signals include echo and near-end crosstalk (NEXT). As a result of these interference signals, the performance of transceivers, in particular the receivers, in a communication system is degraded.
An echo interference signal can be produced by each transmitter contained within the same transceiver as a given receiver. Echo interference signals 302-316 encountered by respective receivers R1-R8 (of transceivers 108-122) are shown in FIG. 3. Echo interference signals 302-316 appear as noise to receivers R1-R8, which are attempting to detect a direct communication signal (e.g., a data symbol) from a transmitter T1-T8 connected at the opposite end of the communication channel. Accordingly, communication signals received by receivers R1-R8 of transceivers 108-122 may experience signal distortion due to echo interference signals 302-316.
NEXT is an interference signal that results from capacitive coupling of signals from a near-end transmitter to the input of a receiver. For example, NEXT interference signals 402-406 encountered by receiver R1 of transceiver 108 are shown in FIG. 4. NEXT interference signals 402-406 appear as noise at the input of receiver R1, which is attempting to detect a direct communication signal from transmitter T5 of transceiver 116. Each of receivers R1-R8 of transceivers 108-122 may encounter the same effect, and accordingly the communication signals received by receivers R1-R8 may also experience signal distortion due to NEXT interference signals. FIG. 5 shows an example time domain representation of echo and NEXT interference signals encountered by receiver R1 of transceiver 108. Echo and NEXT interference signals caused by a reflection due to impedance mismatch at hybrid circuit 132 and connector 148 are identified as the high voltage responses close to zero time.